1. Field of the Invention
The present invention relates to a voltage-measuring device in particular, to a voltage-measuring device for measuring voltage of an in-vehicle high voltage battery that consists of a plurality of rechargeable batteries connected in series with each other.
2. Description of the Related Art
Recently, a hybrid car (hereafter referred to as HEV) that runs using an electric motor together with the engine has become widespread. This HEV includes a low voltage battery for starting engine, and a high voltage battery for driving the electric motor. The high voltage battery consists of a plurality of rechargeable batteries, such as a nickel-hydrogen battery or a lithium battery connected in series with each other.
After repeating charge and discharge, a state of charge (SOC) of each rechargeable battery of the high voltage battery varies. Regarding the charge and discharge of the battery, from a viewpoint of durability of each rechargeable battery and safety, when the rechargeable battery having the highest SOC reaches the upper limit SOC, the charge cycle stops, and when the rechargeable battery having the lowest SOC reaches the lower limit SOC, the discharge cycle stops. Regarding HEV, battery energy is added to a gasoline engine when going up a hill, and the energy is regenerated and stored in the battery. Therefore, the voltage of the high voltage battery often changes by the state of running. Thus, it is necessary to always observe the voltage of a high-voltage battery while the HEV is running.
Japanese published patent application No. Hei 11-176480 discloses a voltage-measuring device that measures voltage of each rechargeable battery that composes the high voltage battery. In this voltage-measuring device, a main battery (high voltage-battery) is divided into a plurality of modules, and after measuring a voltage of each module, the device measures a total voltage of the entire main battery. Then, the device judges whether the voltage of each module and the total voltage of the main battery are abnormal or not. If both of the voltages are normal, the device repeats the processing, and if both of the voltages are abnormal, the device informs of the battery abnormality, and stops the charge and discharge cycle.
The voltage measuring device described above measures the voltage of each module, then measures the total voltage of the main battery, and judges whether both of the voltages are abnormal or not. Therefore, there is a problem of taking time to the detection of abnormality. Further, when a big change of a voltage is measured at a certain module compared with other modules, the judgment whether it is caused by the change of the current value that the main battery supplies (for instance, change of the running state of the HEV) or by abnormality of the rechargeable battery is late.
Accordingly, an object of the present invention is to provide a voltage-measuring device to rapidly detect an abnormality of a high voltage battery consisting of rechargeable batteries connected in series with each other when judging the abnormality.